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Walmart installed thin-film solar cells on this store in Mountain View, Calif. The company plans to put similar panels on 20-30 more stores.Photo: WalmartWhen Walmart announced on Monday that it would install 15 megawatts' worth of solar arrays on as many as 30 of its stores in California and Arizona, it set out to shape the solar market in more ways than one.

The reason? The world's biggest retailer specified that many of the new solar installations should use thin-film photovoltaic panels. Thin-film solar cells are printed or deposited on glass or flexible materials. And although they are less efficient at converting sunlight into electricity, they can be produced at a lower cost than traditional crystalline silicon solar cells.Thin-film solar currently accounts for just about 20 percent of the solar market. The most technologically advanced versions have had a difficult time grabbing market share due to competition with low-cost Chinese crystalline silicon manufacturers and a recession that has dried up investor funding.

Solar panelsWalmart installed thin-film solar cells on this store in Mountain View, Calif. The company plans to put similar panels on 20-30 more stores. When Walmart announced on Monday that it would install 15 megawatts' worth of solar arrays on as many as 30 of its stores in California and Arizona, it set out to shape the solar market in more ways than one.